1. Field of the Invention
The invention relates to power-management methods for electronic systems, and more particularly to power-management methods for electronic systems having Advanced Configuration and Power Interface (ACPI).
2. Description of the Related Art
A method for lowering power consumption in unused computers, sub-systems and their peripheral devices and increasing battery efficiency, which is very important for computer systems, especially for portable computers, such as notebook computers, which often rely on power supply from batteries. Therefore, an electronic system normally employs power management, and there are lots of standards of power management, such as the Advanced Configuration and Power Interface (ACPI) specification. The ACPI is a power management open standard developed by Intel, Microsoft and Toshiba, and provides appropriate power and optimum working frequency in order to balance power saving and efficiency. It should be noted that ACPI is an interface shared between the software (operation system, OS) and hardware, and power management herein is OS-directed instead of BIOS-directed, which is more beneficial for manufacturers when integrating their standards with each other.
Typically, the OS determines whether to enter the power saving states in accordance with the configuration set by some application programs or users. The ACPI specification defines different power states which include a working state (G0), a sleeping state (G1), a software off state (G2), and hardware off state (G3). Normally, a system usually switches between a working state (G0) and a sleeping state (G1), wherein the system further has a power-saving level supported by devices and a power-saving level supported by central processor units. The processor power states (including C0, C1, C2, . . . , Cn state) of ACPI are all defined in the working state (G0). The processor normally executes instructions in the C0 state, and the OS may set the central processor unit to a low-power state (such as the C1, C2, . . . , CN state) when the system is idle for a period of time.
The low power states of the processor defined by ACPI include the C1, C2, C3 and C4 states, wherein the C2 state has lower power consumption than the C1 state, the C3 state has lower power consumption than the C2 state, and the C4 state has lower power consumption than the C3 state. In the C2 state, the processor power does not process any instruction, but monitors the access operation of the Bus Masters. The Bus Master is a unit for controlling buses in a computer system, such as the USB controller, PCI controller, etc. In the C3 state, the clocking of the central processor unit stops, and the central processor unit does not monitor the access operation of the Bus Masters. The voltage of the central processor unit in the C4 state is lower compared with the C3 state, so that the central processor unit has much lower power consumption in the C4 state.
When the operation system of the electronic system detects that the electronic system has no operation for a predetermined period, the operation system requires the central processor unit to enter the C3 or C4 state, so that the power consumption of the electronic system can be reduced.
In the C2 state, if an interrupt event occurs or the central processor unit is requested to process instructions, the central processor unit returns to the C0 state from the C2 state. In the C3 or C4 state, if an interrupt event occurs, the central processor unit is woken, and returns to the C0 state. If the bus master issues an accessing request, the central processor unit is woken, and returns to the C0 state from the C3 or C4 state.